Francesco Princivalle

Cation ordering and structural variations with temperature in MgAl2O4 spinel: An X-ray single-crystal study

Abstract The equilibrium distribution of Mg and Al between the tetrahedral and octahedral sites of a flux grown, stoichiometric MgAl2O4 spinel was investigated between 600 and 1100 °C by single-crystal X-ray diffraction of quenched samples. The cation distribution for both ordering and disordering runs was obtained by minimizing accurate crystallographic parameters and effective ionic radii. Along with the variation of the degree of inversion from 0.18 to 0.29 between 600 and 1100 °C, both unit cell and oxygen positional parameter decreased linearly. Multiple non-linear least-squares fit of our data with the thermodynamic model of O’Neill and Navrotsky (1983) gave α = 23 ± 2 KJ/mol and β = 13 ± 4 KJ/mol. The influence of both cation inversion and thermal expansion on T-O and MO bond length variation was determined by comparison of our data with previous in situ studies. In the thermal interval investigated, the inversion accounts for change of -0.014 Å for T-O and +0.007 Å for M-O. Mean linear polyhedral thermal expansion coefficients of 6.5 × 10-6 °C-1 and 8.9 × 10-6 °C-1 were calculated for T and M sites, respectively.

Microwave generated solid dispersions containing Ibuprofen.

The purpose of this study was to apply the attractive technique of the microwaves irradiation (MW) for the preparation of solvent-free solid dispersions (SD). In particular, the microwave technology has been considered in order to prepare an enhanced release dosage form for the poorly soluble drug Ibuprofen (IBU), employing PVP/VA 60/40 (PVP/VA 64) and hydroxypropyl-beta-cyclodextrin (HP-beta-CD) as hydrophilic carriers. Their physico-chemical characteristics and dissolution properties were compared to the corresponding physical mixtures and the drug alone. The results of physico-chemical characterization attested a correspondence of the solid state of the drug before and after irradiation treatment and that an amorphous form of the drug was obtained. This result, together with the presence of the hydrophilic polymers determined a remarkable enhancement of the in vitro dissolution rate of the drug suggesting that the microwave technique could be considered as a new and interesting method to prepare drug-polymer systems.

Evidence of mantle metasomatism and heterogeneity from peridotite inclusions of northeastern Brazil and Paraguay

Abstract Textural-petrographic features, major and trace elements and mineral chemistry are reported for spinel-peridotite xenoliths from northeastern Brazil (NE-suite) and Paraguay (PY-suite). Variation trends defined by bulk rock and mineral chemistry are consistent with the effects of variable degree of high-pressure melting for both suites. The PY-suite strongly differs from the NE-suite by its very high contents of incompatible elements. This different whole-rock chemistry is related to the presence of glassy-microcrystalline blebs which are interpreted as remains of pre-existing hydrous phases of metasomatic origin. The difference between the two suites is also recorded in the chemistry of pyroxenes and spinels, which develop along different variation patterns. Although the suites partially overlap in their ranges of mg opx (Mg/(Fe 2+ + Fe 3+ ) ratio in orthopyroxenes), the PY-suite is, on the average, more mg opx rich (residual) than the NE-suite. Within the PY-suite a rough positive correlation exists between degree of residuality and degree of metasomatic effects. The sharp differences between the NE-suite and the PY-suite imply mantle heterogeneity on regional scale, whereas the variability within each suite is essentially related to different degrees of melting and/or metasomatism and imply mantle heterogeneity on local scale.

Kinetics of cation ordering in synthetic MgAl2O4 spinel

Abstract The time and temperature evolution of intracrystalline Mg-Al exchange in synthetic MgAl2O4 spinel was studied by single-crystal X-ray diffraction of quenched samples, with the aim of contributing to rock cooling-rate estimation. Flux-grown, homogeneous crystals (0.1 mm in size) were annealed at 1000°C and then were isothermally ordered at 900, 800, and 700°C for 10 seconds to 40 days. The cation ordering process was investigated by measuring the spinel O atom positional parameter u, which had been previously demonstrated to be closely related to the inversion x at equilibrium by the linear equation (R2 = 0.995): x = 21.396 - 80.714 u. From x = 0.27 after annealing runs, the inversion decreased with time for all of the three ordering runs, and equilibrium was reached after about 4 min at T = 900°C (x = 0.25), about 55 min at T = 800°C (x = 0.23), and about 700 min at T = 700°C (x = 0.21). The Mueller kinetic model was satisfactorily applied to the experimental data. Solution of Mueller’s integral gave the kinetic ordering constants K900 = 1.12 ± 0.57, K800 = 0.112 ± 0.047, and K700 = 0.0171 ± 0.0045 min-1, corresponding to t1/2 of 0.6, 6.2, and 40.5 min, respectively. The linear dependence of K with temperature (R2 = 0.99) was observed, and may be expressed by the Arrhenius equation: ln K = 20.189 - 23722/T (K) An activation energy of 197 ± 22 kJ/mol was obtained for the intracrystalline Mg-Al ordering reaction, which compares well with the value of 217 kJ/mol reported in the recent literature for Mg- Fe3+ exchange in spinels

Formulation and Evaluation of Vinylpyrrolidone/Vinylacetate Copolymer Microspheres with Carbamazepine

Vinylpyrrolidone/vinylacetate copolymer (VP/VAc) was used for the enhancement of dissolution rate of carbamazepine, an antiepileptic drug characterized by very low water solubility. Microspheres containing different drug-to-polymer ratios were prepared using a solvent-evaporation technique and their physical characterizations were carried out by differential scanning calorimetry, x-ray diffractometry, and scanning electron microscopy. Through the solubilization kinetics and dissolution rates studies, the in vitro drug availability of the microspheres was evaluated. These results showed that the dissolution of carbamazepine in gastrointestinal fluids from all of the prepared formulations was increased with respect to the drug itself. However, the best dissolution parameters were obtained from 1:10 w/w system because of the presence of the drug in amorphous form and its efficient encapsulation.

Temperature, composition, and fO2 effects on intersite distribution of Mg and Fe2+ in olivines

Single-crystal X-ray structure refinement of natural olivines equilibrated at high temperature under controlled oxygen fugacity (fO2) conditions, coupled with a structure-energy model were used to establish the influence of T, fO2 and bulk chemistry on intracrystalline disorder.The results are: 1) The kD (kD = [(Fe M1·Mg M2)/(Fe M2·Mg M1)]) factor describing site population on M1, M2 polyhedra increases from values lower than 1 at T below 400–600° C (depending on composition) to values higher than 1 at higher temperature. 2) The increase of kD with T is quite regular. 3) At constant temperature and pressure, kD increases with increasing fayalite content in the mixture; 4) Contrary to previous observations (Will and Nover 1979; Nover and Will 1981) varying fO2, within the stability range of the substance, has a negligible influence on intracrystalline disorder. 5) As ancillary results, the model confirms the defect scheme of Nakamura and Schmalzried (1983) for the investigated solids. Moreover, it shows that cationic vacancies are always created on M 1 site at the expense of Mg ions, while trivalent iron is always stabilized on M2 site. This explains the marked anisotropies observed in Fe-Mg interdiffusion (Buening and Busek 1973; Misener 1974; Schock et al. 1989).

Comparative compressibility and structural behavior of spinel MgAl2O4 at high pressures: The independency on the degree of cation order

Abstract The equation of state and the crystal structure evolution with pressure were determined for two single crystals of pure natural MgAl2O4 spinels with different degrees of order. The two samples studied were cut from a larger single crystal and one of them was experimentally disordered at high temperature. The two crystals, showing an inversion parameter x of 0.27 and 0.15 at ambient conditions, were loaded together in a diamond anvil cell and their unit-cell edge was measured up to about 7.5 GPa at 14 different pressures. The unit-cell volume, V0, the bulk modulus, KT0, and its first pressure derivative, K′, were simultaneously refined using a third-order Birch-Murnaghan equation of state, giving the following coefficients: V0 = 529.32(2) Å3, KT0 = 193(1) GPa, K′ = 5.6(3) for the ordered sample and V0 = 528.39(2) Å3, KT0 = 192(1) GPa, K′ = 5.4(3) for the disordered one. Complete intensity data were collected at 0, 0.44, 2.92, 7.34, and 8.03 GPa in a separate experiment. For the ordered and disordered samples the oxygen atomic coordinate u remains practically unchanged inside the investigated pressure range with an average value of 0.2633(5) and 0.2614(2), respectively. As a consequence, the polyhedral compressibilities are similar and are not influenced by the Mg/Al distribution over the two crystallographic sites. This also suggests that pressure has little or no influence on the degree of order in the MgAl2O4 spinel.

Study of the solid state of carbamazepine after processing with gas anti-solvent technique

The purpose of this study was to investigate the influence of supercritical CO2 processing on the physico-chemical properties of carbamazepine, a poorly soluble drug. The gas anti-solvent (GAS) technique was used to precipitate the drug from three different solvents (acetone, ethylacetate and dichloromethane) to study how they would affect the final product. The samples were analysed before and after treatment by scanning electron microscopy analysis and laser granulometry for possible changes in the habitus of the crystals. In addition, the solid state of the samples was studied by means of X-ray powder diffraction, differential scanning calorimetry, diffuse reflectance Fourier-transform infrared spectroscopy and hot stage microscopy. Finally, the in vitro dissolution tests were carried out. The solid state analysis of both samples untreated and treated with CO2, showed that the applied method caused a transition from the starting form III to the form I as well as determined a dramatic change of crystal morphology, resulting in needle-shaped crystals, regardless of the chosen solvent. In order to identify which process was responsible for the above results, carbamazepine was further precipitated from the same three solvents by traditional evaporation method (RV-samples). On the basis of this cross-testing, the solvents were found to be responsible for the reorganisation into a different polymorphic form, and the potential of the GAS process to produce micronic needle shaped particles, with an enhanced dissolution rate compared to the RV-carbamazepine, was ascertained.

Structural parameters of chromite included in diamond and kimberlites from Siberia: A new tool for discriminating ultramafic source

Abstract Siberian chromite included in diamond, kimberlite, and spinel peridotite as well as Cr-spinel from garnet-spinel peridotite have been studied by single-crystal X-ray diffraction and electron microprobe analysis. Cell edges and oxygen positional parameters, u, of chromite in diamond and in kimberlite are comparable (cell edge, 8.3249-8.3390 Å; u, 0.26175-0.26213). The structural parameters of chromite in the spinel peridotite are similar to those of chromite grains from ophiolitic complexes, and those of Cr-spinel from garnet-spinel peridotite are comparable to those of Cr-spinel in lherzolitic mantle xenoliths. With the exception of the chromite in garnet-spinel peridotite, all analyzed spinels have a high Cr content. Recasting the chemical analyses according to spinel stoichiometry reveals negligible or no Fe3+. Chrome spinel may be present in heavy concentrates derived from serpentinized mafic and ultramafic rocks as the sole surviving primary mineral and, as such, their particular structural and chemical parameters may represent a new prospecting tool for discriminating the ultramafic source.

Influence of temperature and composition on Mg-Fe2+ intracrystalline distribution in olivines

SummaryMicroprobe analyses and X-ray crystal structure refinement of terrestrial and lunar olivines are compared, in order to investigate the influence of temperature and composition on Mg-Fe2+ distribution between M1 and M2 sites.Quenching and heating experiments show that, for the same composition, a temperature increase causes an increase in the (Fe2+/Mg)M1/(Fe2+/Mg)M2 ratio (KD. Such experiments also demonstrate that the intracrystalline Mg-Fe2+ ordering in Ml and M2 sites is very fast and the calculated KD values therefore depend on cooling rate (C.R.), i.e., high KD values = high CR.. On the other hand, for the same cooling rate and temperature, Fe-rich olivine has a KD value higher than that of Mg-rich olivine, since Fe2+ is preferentially ordered in M1 site. Oxygen fugacity variations (QFI and QFM buffers) do not appreciably influence KD.The closure temperature of Mg-Fe2+ ordering between M1-M2 sites of olivine is probably lower than that estimated for orthopyroxene (c. 550–600°C; Saxena et al., 1989), owing to a lesser degree of “non-equivalence” between MI and M2 sites. The highest temperature that can be detected by KD values of natural olivines is 700–800 °C.ZusammenfassungMikrosondenuntersucbungen und Strukturverfeinerungen an terrestrischen und lunaren Olivinen werden verglichen, um den Einfluß von Temperatur und Zusammensetzung auf die Mg-Fe2+ Verteilung in der MI und M2 Position zu untersuchen. Experimentelle Untersuchungen belegen, daß bei gleichbleibender chemischer Zusamensetzung, eine Temperaturzunahme eine Zunahme im KD Verhältnis (Fe2+ /Mg)M1/(Fe2+/M9)M2 bedingt. Diese Experimente zeigen weiters, daß die Ordnung der Mg-Fe2+ Kationen im Gitter in den M 1 und M2 Positionen sehr rasch erfolgt und die berechnete KD Werte daher von der Abkühlungsrate (C.R.) abhängen; z. B. hohe KD-Werte = hohe C. R.Andererseits hat bei gleicher Abkühlungsrate und Temperatur Fe-reicher Olivin einen höheren KD Wert als Mg-reicher Olivin, da Fe2+ bevorzugt in die M1 Position eingebaut wird. änderungen in der Sauerstoff-Fugazität (QFI und QFM Buffer) beeinflussen die KD Werte unwesentlich.Die “Schließungstemperaturen” für die Mg-Fe2+ Ordnung zwischen der M1 und M2 Position in Olivin sind wahrscheinlich niedriger als die von Orthopyroxen (ca. 550–600°C;Saxena et al., 1989); dies wird mit einem niedrigeren Grad an “non-equivalence” von Ml und M2 erklärt. Die höchsten Temperaturen, die an Hand der KD-Werte an natürlichen Olivinen bestimmt wurden liegen bei 700–800°C.